[llvm-bugs] [Bug 164398] [clang] Clang accepts explicit instantiation of a function template with deduced return type and explicit trailing return type (non‑standard)

[LLVM Bugs via llvm-bugs]

Issue	164398
Summary	[clang] Clang accepts explicit instantiation of a function template with deduced return type and explicit trailing return type (non‑standard)
Labels	clang
Assignees
Reporter	Attacker23

    I suspect that **Clang currently accepts a non‑standard form of explicit instantiation** that other major compilers (GCC, MSVC, EDG) correctly reject:

```cpp
struct A {
  char g() { return 'a'; }
  template<class T> auto f(T t) {
    return t + g();
  }
};

template auto A::f<int>(int t) -> decltype(t + g());
// The correct code should probably be:
// template auto A::f<int>(int t);
```

What’s puzzling is the version history:

- **Clang ≤ 16.0.0** correctly rejected this code.  
- **Clang 17.0.1 – 18.1.0** caused a **crash**.  
- **Clang 19.1.0+** now **accepts** the code silently.

It would be helpful to know **which change or patch introduced this behavior**,  
and whether the current acceptance is intentional or a missing diagnostic.

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